The background art of the present invention will be described using an excavator as an example.
For example, as shown in FIG. 3, a general excavator comprises a crawler-type base carrier 1, an upper slewing body 2 mounted on the base carrier 1 so as to be capable of slewing around an axis X perpendicular to the ground, and an excavating attachment 3 attached to the upper slewing body 2. The excavating attachment 3 includes: a boom 4 capable of being raised and lowered; an arm 5 attached to a tip of the boom 4; a bucket 6 attached to a tip of the arm 5; and a boom cylinder 7, an arm cylinder 8, and a bucket cylinder 9 which are respective cylinders (hydraulic cylinders) for actuating the boom 4, the arm 5, and the bucket 6.
FIG. 4 shows an example of a conventional hydraulic circuit for slewing the upper slewing body 2. The circuit includes: a hydraulic pump 10 as a hydraulic pressure source that is driven by an engine not graphically shown; a slewing hydraulic motor 11 which is rotated by hydraulic pressure supplied from the hydraulic pump 10 to drive the upper slewing body 2 to slew it; a remote-control valve 12 as a slewing operation device including a lever 12a to which an operation is applied to input a command for the slewing; and a control valve 13 which is a pilot operated selector valve that can be operated by the remote-control valve 12 and provided between the hydraulic motor 11a and a pair of the hydraulic pump 10 and a tank T.
The lever 12a of the remote-control valve 12 is operated between a neutral position and right and left slewing positions, and the remote-control valve 12 outputs a pilot pressure with a magnitude corresponding to an operation amount of the lever 12a from a port corresponding to an operation direction of the lever 12a. The control valve 13 is switched from a graphically shown neutral position 13a to a left slewing position 13b or a right slewing position 13c by the pilot pressure, thereby controlling respective directions of supply of the hydraulic fluid to the hydraulic motor 11 and of right and left discharge of the hydraulic fluid from the hydraulic motor 11, and a flow rate of the hydraulic fluid. In other words, performed are: switching slewing state, that is, selectively switching to respective states of acceleration (including start-up), steady operation at a constant speed, deceleration, and stop; and controlling slewing direction and slew speed.
The control valve 13 and respective right and left ports of the hydraulic motor 11 are interconnected through a right slewing pipe-line 15 and a left slewing pipe-line 14. Between both slewing pipe-lines 14 and 15, provided are a relief valve circuit 18, a check valve circuit 21, and a communication path 22. The relief valve circuit 18 is provided so as to interconnect the slewing pipe-lines 14 and 15, and the relief valve circuit 18 is provided with a pair of relief valves 16 and 17 having respective outlets which are opposed and connected to each other. The check valve circuit 21 is provided so as to interconnect the slewing pipe-lines 14 and 15 at a position closer to the hydraulic motor 11 than the relief valve circuit 18, and the check valve circuit 21 is provided with a pair of check valves 19 and 20 having respective inlets which are opposed and connected to each other. The communication path 22 interconnects a first portion of the relief valve circuit 18, the first portion located between both relief valves 16 and 17, and a second portion of the check valve circuit 21, the second portion located between both check valves 19. The communication path 22 is connected to the tank T through a make-up line 23 for sucking up hydraulic fluid, and the make-up line 23 is provided with a back pressure valve 24.
In this circuit, when the remote-control valve 12 is not operated, that is, when the lever 12a thereof is at a neutral position, the control valve 13 is kept at the neutral position 13a; when the lever 12a of the remote-control valve 12 is operated to the left or the right from the neutral position, the control valve 13 moves from the neutral position 13a to the left slewing position 13b or the right slewing position 13c in accordance with an operating direction of the lever 12a, by a stroke in accordance with an operation amount of the lever 12a. 
At the neutral position 13a, the control valve 13 blocks both slewing pipe-lines 14 and 15 from the pump 10 to prevent the hydraulic motor 11 from rotation; when switched to the left slewing position 13b or the right slewing position 13c, the control valve 13 allows hydraulic fluid from the pump 10 to be supplied to the left slewing pipe-line 14 or the right slewing pipe-line 15 to thereby bring the hydraulic motor 11 into a slewing-driving state of leftward or rightward rotating to slew the upper slewing body 2. The slewing-driving state includes both an accelerative slewing state including start-up and a steady operation state at a constant rotational speed. Meanwhile, the fluid discharged from the hydraulic motor 11 is returned to the tank T via the control valve 13.
Next will be described deceleration of slewing. For example, in the rightward slewing, i.e., clockwise slewing, upon a deceleration operation applied to the remote-control valve 12, specifically, upon an operation for returning the lever 12a to the neutral position or to the side of the neutral position, the control valve 13 is operated to the side of returning to the neutral position 13a to stop the supply of hydraulic fluid to the hydraulic motor 11 and the return of hydraulic fluid from the hydraulic motor 11 to the tank T, or to reduce a supply flow rate and a return flow rate of the hydraulic fluid. Meanwhile, the hydraulic motor 11 continue its clockwise rotation due to the inertia of the upper slewing body 2, thus raising pressure in the left slewing pipe-line 14 as a meter-out-side line. When the raised pressure reaches a certain value, the relief valve 16 on the left side in the diagram is opened to allow hydraulic fluid in the left slewing pipe-line 14 to flow into the hydraulic motor 11 through the relief valve 16, the communication path 22, the check valve 20 on the right side in the diagram, and the right slewing pipe-line 15 as indicated by a dashed-line arrow in FIG. 4. This gives a braking force due to the action of the relief valve 16 against the hydraulic motor 11 which continues to rotate due to the inertia, thereby decelerating and stopping the hydraulic motor 11. Decelerating and stopping the leftward slewing are similarly performed. On the other hand, when the slewing pipe-line 14 or 15 is subjected to negative pressure during the deceleration, the hydraulic fluid in the tank T is sucked up into the slewing pipe-line 14 or 15 through the make-up line 23, the communication path 22 and the check valve circuit 21, thereby preventing cavitation.
Japanese Patent Application Laid-open No. 2010-65510 discloses an excavator including a circuit as shown in FIG. 4 described above, the excavator further including: a slewing electric motor connected to the hydraulic motor 11; a direct-interconnection selector valve switchable between a direct-interconnection position for directly interconnecting the left and right pipe-lines 14 and 15 and a cutoff position for cutting off the direct interconnection; an electric storage device; and a controller which switches the direct-interconnection selector valve to the direct-interconnection position during slewing deceleration to return motor-discharged fluid to a motor inlet-side and cause the slewing electric motor to perform an electric motor action, wherein the electric storage device stores regenerative power generated by the electric motor action. With this technique, the direct-interconnection selector valve reduces back pressure that acts on a motor outlet-side during slewing deceleration to reduce drag load of the hydraulic motor. This allows efficiency of recovery (in other words, regeneration) of inertial kinetic energy to be improved.
Although the known art described in Japanese Patent Application Laid-open No. 2010-65510 uses a solenoid selector valve as the bypass selector valve, there may be cases where the motor load is required to be reduced by use of not a solenoid selector valve but a hydraulic-pilot-controlled selector valve, for example, in the case of a relatively large flow rate or in the case of requiring an absorption of a shock produced by switching. In such a case, in order to electrically switch the hydraulic-pilot-controlled selector valve, provided are a communication selector valve constituted by a separate solenoid selector valve between a pilot port of the hydraulic-pilot-controlled selector valve and a hydraulic pilot pressure source; the communication selector valve is opened and closed, thus allowing turning on and off the input of the pilot pressure to the hydraulic-pilot-controlled selector valve to be performed.
However, in this case, if a phenomenon such as fixation of a spool of the communication selector valve or the like occurs and causes such a failure that the communication selector valve is prevented from a movement from a pilot pressure supply position, there may be continued a state where pilot pressure is supplied to the hydraulic-pilot-controlled selector valve even after slewing has stopped. Hence, for example, in the case where the direct-interconnection selector valve is made up of the hydraulic-pilot-controlled selector valve, the direct-interconnection selector valve is brought into a state of directly interconnecting both of the pipe-lines, which makes it impossible to prevent the hydraulic motor and the upper slewing body connected thereto from rotation. This generates a fear of allowing an upper slewing body to slew due to its own weight on inclined ground or the like.    Patent Document 1: Japanese Patent Application Laid-open No. 2010-65510